Piles, usually made out of concrete, are generally used to form the foundations of buildings or other large structures. Before using the piles as a foundation for further building work, it is important to test the static load-bearing capacity of each pile. This is generally done by applying a test load to the top of a pile by way of a hydraulic jack braced against a reaction system having a cross-beam that is anchored in place at its ends. The test load is generally measured by monitoring the hydraulic pressure supplied to the jack, and the associated displacement of the pile is measured by using a displacement sensor. Frequently, the displacement of the pile is measured for a number of increasing test loads, each applied for a predetermined time. Because the applied test loads tend to be high, there is a significant danger to operating personnel should the cross-beam or its anchorages fail, particularly if the operating personnel are required to read test values from one or more gauges located on equipment located close to the top of the pile.
Furthermore, because the applied test load has to be maintained and adjusted by operating the jack manually, it is necessary for operating personnel to be in attendance at all times. It is not safe for a single operator to work alone, particularly overnight (the typical time taken to perform a comprehensive static load test can often be as much as 18 hours). Accordingly, the typical method of static load testing is expensive, as well as being slow.
Another disadvantage of the known static load-testing equipment is that the quality of the data obtained is not always consistently good. Typical data required from a static load test are the record of displacement of the pile head and the load applied. Although manual reading and recording of the dial gauges employed in a static load test should not present an insurmountable difficulty in terms of accuracy and regularity, it is the application of the load that generally is the source of poor quality data. This is principally due to the need to attend continuously to a manual hydraulic pump in order to maintain the load with any degree of constancy. A further source of error arises through the use of a pressure gauge to derive the applied test load by way of calibration charts. The accuracy with which the load can be maintained is governed by the resolution with which the gauge can be read. Assuming the operator performing the load control is entirely dedicated and doing his utmost to maintain the load, he may at best be able to read a pressure or load column gauge to 1%. This implies that the load variation is not likely to be better than around +/−2%. This in turn means that the pile head displacement recording of a pile whose elastic shortening alone is about 5 mm, will fluctuate by +/−0.1 mm according to this load variation.
British patent application GB 2323174A teaches a method and apparatus for testing the static load-bearing capacity of a pile. However, this teaching relates to the application of load at the top of the pile by way of a jack braced against a reaction member suitably anchored into the ground, such that the full test load needs to be applied to the foundation under test.
U.S. Pat. No. 4,614,110 and U.S. Pat. No. 5,576,494 teach a method of loading from the bottom of a pile where a load device is positioned between the bottom of a pile and the bottom of the hole in which the pile is located.